Located on the edge of downtown Boise, the Castle Rock Loop is a popular and quick opportunity to experience the Boise foothills. Castle Rock was formed by a volcanic rhyolite flow that permeated the existing sandstone in the area, leaving a rock outcropping along the edge of the Treasure Valley.
Although no longer active, previous volcanic activity at the site generated hot water springs used by local Native Americans for bathing and ritual means. The flowing hot water is the namesake for nearby Warm Springs Avenue, where many historic homes still make use of natural geothermal water for heating.
The hike begins at the historic Old Penitentiary, which is also a common trailhead for nearby Table Rock. The Castle Rock loop heads northwest from the trailhead to downtown, and after a quick 0.3 miles uphill, you'll find yourself with an outstanding view of Boise from the rocky outcrop.
From here you'll have a spectacular view of downtown Boise, the Boise River below, and the Owyhee Mountains in the distance that line the border between Oregon and Idaho.
From Castle Rock the loop trail continues northwest before switchbacking downhill toward the Hot Springs Trail. As the trail swings back to the southeast it runs along Quarry View Park, which has a large grassy lawn and public shelter.
For those seeking a quick experience in the foothills of Boise with a spectacular view, Castle Rock is a great destination.